This invention relates to a loudspeaker apparatus adapted to reproduce audio signals by means of an ultrasonic generating device and a method of driving such a loudspeaker apparatus. It also relates to an audio signal transmitter comprising an ultrasonic generating device.
Loudspeaker apparatus comprising a diaphragm to be oscillated to emit sounds into air and hence to listeners are being popularly used. Loudspeaker apparatus of this type are normally adapted to make the diaphragm to be oscillated by audio signals of an audible band between 20 Hz and 20 KHz so as to emit sound waves directly into air.
More specifically, sound is emitted into air from the diaphragm of a loudspeaker apparatus that is adapted to make the diaphragm to be oscillated by audio signals of an audible band so that the emitted sound is spread, or propagated, through air from the diaphragm that operates as the center of propagation. Such a loudspeaker apparatus is useful for emitting sound into a large space.
However, such a loudspeaker apparatus cannot be used to emit sound to one or more than one specific listeners.
Headphones and earphones are used to allow individuals to listen to a reproduced sound. Such headphones and earphones also comprise a diaphragm that is oscillated by audio signals of an audible band and the sound emitted from the diaphragm is spread into air. A headphone set or an earphone set has to be borne by the head or the ears, whichever appropriate, of the user with the loudspeaker units of the set held in a closed state so that the listener may hear the reproduced sound secretly.
It is an object of the present invention to provide a loudspeaker apparatus adapted to ultradirectionally emit sound by means of a novel drive system and a method of driving such a loudspeaker apparatus.
Another object of the present invention is to provide a loudspeaker apparatus adapted to secretly emit sound and a method of driving such a loudspeaker apparatus.
Still another object of the present invention is to provide a loudspeaker apparatus adapted to emit different sounds that can be heard at respective different locations and a method of driving such a loudspeaker apparatus.
Still another object of the present invention is to provide a loudspeaker apparatus that can set up a phone source at any location and a method of driving such a loudspeaker apparatus.
A further object of the present invention is to provide an audio signal transmitting/receiving apparatus adapted to transmit and receive audio signals with an increased degree of secrecy.
According to an aspect of the invention, the above objects and other objects are achieved by providing a loudspeaker apparatus comprising a modulator for frequency-modulating an audio signal to a signal of a frequency band at least higher than the audible frequency band and at least an ultrasonic generating device adapted to be driven by the output signal of said modulator.
Said modulator frequency-modulates said audio signal into a first signal frequency-modulated on the basis of a first frequency and a second signal frequency-modulated on the basis of a second frequency different from the first frequency.
Preferably, a loudspeaker apparatus according to the invention comprises a plurality of ultrasonic generating devices, said first signal being supplied to part of said plurality of ultrasonic generating devices, said second signal being supplied to the rest of said plurality of ultrasonic generating devices.
Preferably, a loudspeaker apparatus according to the invention further comprises a differential processing section for differentiating said audio signal and said modulator includes a first modulating section and a second modulating section, the output signal of said differential processing section being supplied to either said first or said second modulating section, a signal obtained by inverting the polarity of said output signal of said differential processing section being supplied to the other modulating section.
Preferably, a loudspeaker apparatus according to the invention further comprises a first circuit section for supplying a signal obtained by shifting the DC level of said output signal from said differential processing section to either said first or said second modulating section and a second circuit section for supplying a signal obtained by inverting the polarity and shifting the DC level of said output signal from said differential processing section to the other modulating section.
Preferably, a loudspeaker apparatus according to the invention further comprises a preprocessing circuit for preprocessing said audio signal and said modulator includes a first modulating section for amplitude-modulating the output signal of said preprocessing circuit, using the output signal of said first modulating section as a carrier wave, and a second modulating section for amplitude-modulating the output signal of said preprocessing circuit, using the output signal of said second modulating section as a carrier wave.
Preferably, a loudspeaker apparatus according to the invention further comprises a correction filter arranged between said modulator and said ultrasonic generating device. Said correction filter suppresses the resonance frequency components of the ultrasonic generating device out of said output signal of said modulator.
According to the invention, there is also provided a loudspeaker apparatus comprising a modulator including a first modulating section and a second modulating section, either said first or said second modulating section being adapted to be supplied with an audio signal, the other modulating section being adapted to be supplied with a signal obtained by inverting said audio signal, said audio signal being frequency-modulated into a signal of a frequency band at least higher than the audible frequency band, and an ultrasonic generating section adapted to be driven by the output signal of said modulator, said ultrasonic generating device including a first generating section comprising a plurality of ultrasonic generating devices adapted to be driven by the output signal of said first modulating section and a second generating section comprising a plurality of ultrasonic generating devices adapted to be driven by the output signal of said second modulating section.
According to another aspect of the invention, there is also provided an audio signal transmitting/receiving apparatus comprising a modulator for frequency-modulating a carrier wave by a signal obtained by differentiating an audio signal, an ultrasonic generating section adapted to be driven by the output signal of said modulator, a microphone for detecting the sound wave output from said ultrasonic generating section and an arithmetic section for arithmetically processing the output signal of said microphone by using an inverse cosine function.
Preferably, in an audio signal transmitting/receiving apparatus according to the invention, said microphone detects the sound wave of the audible frequency band output from said ultrasonic generating section.
According to still another aspect of the invention, there is provided a method of driving a loudspeaker apparatus comprising at least an ultrasonic generating device, said method comprising a step of frequency-modulating the input audio signal into a signal of a frequency band at least higher than the audible frequency band and a subsequent step of driving the ultrasonic generating device by said signal obtained by frequency-modulation.
In a method of driving a loudspeaker apparatus according to the invention, said audio signal is frequency-modulated into a first signal on the basis of a first frequency and a second signal on the basis of a second frequency different from said first frequency in said frequency-modulating step.
The above and other objects and the advantages of the invention will become apparent from the following description to be made by referring to the accompanying drawings that illustrate preferred embodiments of the invention.